HYDROGEN-BONDING .62. EFFECT OF SODIUM-FLUORIDE ON THE DISSOLUTION OF1-METHYL-4,5-DICARBOXYIMIDAZOLE

The IR spectrum and pK(a) values of 1-methyl-4,5-dicarboxyimidazole (H (2)MDCI) support a zwitterionic structure with one acidic hydrogen on the non-methylated nitrogen (N3) and the second acidic hydrogen involv ed in a covalent three-center hydrogen bond between carboxylate groups ; the N3 hydrogen is involved in strong intermolecular hydrogen bondin g. On half-neutralization the carboxylate hydrogen is removed, and the remaining N3 hydrogen acts as a weak hydrogen bond donor. The solubil ity of the diacid in H2O is increased by the addition of fluoride ion. The stoichiometry of the dissolution process is H(2)MDCI + 4F(-) = MD CI(2-) + 2HF(2)(-). The unexpected removal of both acidic hydrogens in the presence of undissociated diacid is driven by strong chelation of sodium ion by MDCI(2-). Evaporation of a 3:1 NaF to H(2)MDCI solution yields Na(2)MDCI and NaH2F3; with excess NaF, Na(2)MDCI and NaHF2 are obtained. With quantities of NaF below a 3:1 stoichiometry, various m ixtures of H(2)MDCI, NaHMDCI, and Na(2)MDCI are obtained on evaporatio n.